Graded-refractive-index optical plastic material and method for its production

ABSTRACT

A graded-refractive-index optical plastic material composed of an amorphous fluoropolymer and at least one material which differs from the polymer in refractive index by at least 0.001, wherein the material is so distributed as to have a concentration gradient in a specific direction, and a method for its production.

TECHNICAL FIELD

[0001] The present invention relates to a graded-refractive-indexoptical plastic material (hereinafter sometimes referred to as opticalplastic material for short) having high transparency and high heatresistance simultaneously, which conventional optical plastics hardlyever attained, and a method for its production.

[0002] The optical plastic material of the present invention may be alight transmission medium such as an optical fiber, or a body materialof a light transmission medium such as a preform of an optical fiber.

[0003] A light transmission medium which is the optical plastic materialof the present invention is free from light scattering and verytransparent to light at wavelengths within a wide range from ultravioletlight to near infrared light, since it is made of an amorphous resin,therefore it is useful for optical systems for light of variouswavelengths. In particular, the optical plastic material of the presentinvention provides a light transmission medium with small losses atwavelength of 1300 nm and 1550 nm, at which a trunk vitreous silicafiber is used in the field of optical communication.

[0004] A light transmission medium which is the optical plastic materialof the present invention has heat resistance, chemical resistance,humidity resistance and nonflammability enough to withstand severe useconditions, for example, in an engine room of an automobile.

[0005] A light transmission medium which is the optical plastic materialof the present invention is useful as various graded-refractive-indexlight transmission medium such as a graded-refractive-index opticalfiber, a rod lens, an optical waveguide, an optical decoupler, awavelength multiplexer, a wavelength demultiplexer, an opticalattenuator, a light switch, an optical isolator, a light transmittingmodule, an light receiving module, a coupler, an optical deflector andan optical integrated circuit. Graded-refractive-index means a regionwherein the refractive index of a light transmission medium variescontinuously in a specific direction. For example, agraded-refractive-index optical fiber shows a refractive index profilethat the refractive index parabolically decreases from the center of thefiber along the radii.

[0006] When the optical plastic material of the present invention is abody material-of a light transmission medium, it is spun, for example,by hot drawing to prepare a light transmission medium such as agraded-refractive-index optical fiber.

BACKGROUND ART

[0007] Heretofore, as resins for graded-refractive-index plastic lighttransmission medium, optical plastics represented by methyl methacrylateresins, tetrafluoroethylene resins disclosed in WO94/04949 and vinylfluoride resins have been proposed.

[0008] With respect to stepped-refractive-index plastic optical fibers,many proposals have been made to use optical plastics such as a methylmethacrylate resin, a styrene resin, a carbonate resin and a norborneneresin for a core and a fluoropolymer cladding. Japanese UnexaminedPatent Publication No. 244007/1990 proposes use of a fluoropolymer coreand a fluoropolymer cladding.

[0009] The present invention provides an optical plastic material havingheat resistance, humidity resistance, chemical resistance andnonflammability required for applications to an automobile, an officeautomation (OA) equipment, an electrical appliance and the like, whichlight transmission medium made of a methyl methacrylate resin or anorbornene resin have never attained.

[0010] Further, the object of the present invention is to provide anovel optical plastic material which is useful for ultraviolet light(wavelength from 200 nm to 400 nm) and near infrared light (wavelengthfrom 700 nm to 2500 nm), which are unavailable to light transmissionmedium made of a methacrylate resin, a carbonate resin and a norborneneresin, and has low light transmission losses in a wide transmission zoneand a method of its production.

DISCLOSURE OF INVENTION

[0011] The present inventors have conducted extensive researches takingthe above-mentioned problems into consideration, and consequently foundthat a fluoropolymer which substantially has no C—H bond is the mostsuitable to provide heat resistance, humidity resistance, chemicalresistance and nonflammability and eliminate C—H bonds (namelycarbon-hydrogen bonds) which absorb near infrared light. Thefluoropolymer has C—F bonds (namely carbon-fluorine bonds) instead ofC—H bonds.

[0012] When a substance is exposed to light, a certain interatomic bondabsorbs preferentially light of wavelength resonant with its stretchingvibration and deformation vibration. Conventional polymeric materialsused for plastic optical fibers are mostly compounds having C—H bonds.Such polymeric materials which basically have C—H bonds show the mainabsorption bands at a shorter wavelength (3400 nm) in the infraredregion, since a hydrogen atom is so light as to easily vibrate.Accordingly, in the near infrared to infrared region (from 600 to 1550nm), which is the wavelength region of a light source, relatively lowerharmonic absorption peaks due to the stretching vibration of C—H bondsappears at intervals and they are greatly responsible for absorptionloss.

[0013] If hydrogen atoms are substituted with fluorine atoms, theseharmonic absorption peaks shift to longer wavelengths, and the amount ofabsorption in the near infrared region decreases. In the case of a PMMA(polymethyl methacrylate) having C—H bonds, the absorption lossattributable to the C—H bonds is estimated theoretically to be 105 dB/kmat a wavelength of 650 nm and at least 10000 dB/km at a wavelength of1300 nm.

[0014] On the contrary, in the case of a material in which hydrogenatoms are substituted with fluorine atoms, there is substantially noabsorption loss at a wavelength of 650 nm, and the absorption loss at awavelength of 1300 nm, which is between the sixth and the seventhovertones, is in the order of 1 dB/km and therefore negligible. For thisreason, we propose to use a compound having C—F bonds.

[0015] It is also preferred to eliminate functional groups such as acarboxyl group and a carbonyl group which inhibit heat resistance,humidity resistance, chemical resistance and nonflammability. Further,since the presence of a carboxyl group-results in absorption of nearinfrared light, and the presence of a carbonyl group results inabsorption of ultraviolet light, it is preferred to eliminate thesegroups. In addition, in order to reduce a transmission loss due to lightscattering, it is important to use an amorphous polymer.

[0016] In the case of a stepped-refractive-index optical fiber,multimodal light is propagated in it, by being reflected on theinterface between the core and the cladding. Therefore, mode dispersionoccurs, and as a result, the transmission zone decreases. However, agraded-refractive-index optical fiber hardly causes mode dispersion, andtherefore, the transmission zone increases.

[0017] The present inventors found out an optical plastic materialcomposed of an amorphous fluoropolymer which substantially has no C—Hbond, especially a fluoropolymer having a cyclic structure on its mainchain, and a material which differs from the polymer in refractiveindex, wherein the concentration of the material shows a gradient in aspecific direction and a method of its production for the first time,and achieved the following present inventions (1) to (2).

[0018] (1) A graded-refractive-index optical plastic material composedof an amorphous fluoropolymer (a) which substantially has no C—H bondand at least one material (b) which differs from the fluoropolymer (a)in refractive index by at least-0.001, wherein the material (b) is sodistributed in the fluoropolymer (a) as to have a concentration gradientin a specific direction.

[0019] (2) A method of producing a graded-refractive-index opticalplastic material, which comprises melting an amorphous fluoropolymer (a)which substantially has no C—H bond, injecting at least one material (b)which differs from the fluoropolymer (a) in refractive index by at least0.001, or the fluoropolymer (a) containing the material (b) at thecenter of the melt of the fluoropolymer (a), and molding the melt whileor after diffusing the material (b) to form a region wherein therefractive index varies continuously. Heretofore, a tetrafluoroethyleneresin, a perfluoro(ethylene-propylene)resin, a perfluoroalkoxy resin, avinylidene fluoride resin, an ethylene-tetrafluoroethylene resin, achlorotrifluoroethylene resin and the like have been widely known asfluoropolymers. However, since these fluoro resins are crystalline andcauses light scattering, they are poor in transparency and unfavorableas materials for a plastic light transmission medium.

[0020] In contrast to these fluoro resins, amorphous fluoropolymers arefree from light scattering due to crystals and therefore, are excellentin transparency. The fluoropolymer (a) of the present invention is by nomeans limited so long as it is an amorphous fluoropolymer having no C—Hbond, however, a fluoropolymer having a cyclic structure on itsmain-chain is preferred. As a fluoropolymer having a cyclic structure onits main chain, fluoropolymers having a fluorine-containing alicyclicstructure, a fluorine-containing cyclic imide structure, afluorine-containing triazine ring structure or a fluorine-containingaromatic ring structure are preferred. Among fluoropolymers having afluorine-containing alicyclic structure, those having afluorine-containing alicyclic ether structure are more preferred.

[0021] A fluoropolymer having a fluorine-containing alicyclic structureis less likely to undergo orientation of polymer molecules, when hotdrawn or melt spun into fibers, as compared with fluoropolymers having afluorine-containing cyclic imide structure, a fluorine-containingtriazine ring structure or a fluorine-containing aromatic ringstructure. Consequently, it does not cause light scattering, thereforeis a more preferred polymer.

[0022] The viscosity of the fluoropolymer (a) in a molten state ispreferred to be from 103 to 105 poise at a melt temperature of from 200°C. to 300° C. If the melt viscosity is too high, not only melt spinningis difficult, but also diffusion of the material (b) required for agraded refractive index, hardly takes place, and formation of a gradedrefractive index is difficult. On the other hand, if the melt-viscosityis too low, there are practical problems. Namely, in the case of use asa light transmission medium in an electronic equipment or an automobile,it becomes soft upon exposure to a high temperature, and the lighttransmission performance becomes poor.

[0023] The number-average molecular weight of the fluoropolymer (a) ispreferably from 10,000 to 5000,000, more preferably from 50,000 to1000,000. A too small molecular weight can interfere with heatresistance, and too a large molecular weight makes it difficult to forma graded-refractive-index light transmission medium, such beingunfavorable.

[0024] As a polymer having a fluorine-containing alicyclic structure,those obtainable by polymerization of a monomer having afluorine-containing cyclic structure, and a polymer having afluorine-containing alicyclic structure on its main chain which isobtainable by cyclic polymerization of a fluorine-containing monomerhaving at least two polymerizable double bonds are preferred.

[0025] Polymers having a fluorine-containing alicyclic structure ontheir main chains which are obtainable by polymerization of monomershaving a fluorine-containing alicyclic structure are reported byJapanese Examined Patent Publication No. 18964/1988 and the like.Namely, polymers having a fluorine-containing alicyclic structure ortheir main chains are obtained by homopolymerization of a monomer havinga fluorine-containing alicyclic structure such asperfluoro(2,2-dimethyl-1,3-dioxole), or by polymerization of such amonomer with a radical polymerizable monomer such astetrafluoroethylene, chlorotrifluoroethylene and perfluoro(methyl vinylether).

[0026] Further, polymers having a fluorine-containing alicyclicstructure on their main chains which are obtainable by cyclicpolymerization of a fluorine-containing monomer having at least twopolymerizable double bonds are reported by Japanese Unexamined PatentPublication No. 238111/1988, Japanese Unexamined Patent Publication No.238115/1988 and the like. Namely, polymers having a fluorine-containingalicyclic structure on their main chains are obtained by cyclicpolymerization of a monomer such as perfluoro(allyl vinyl ether) andperfluoro(butenyl vinyl ether), or copolymerization of such a monomerwith a radical polymerizable monomer such as tetrafluoroethylene,chlorotrifluoroethylene and perfluoro(methyl vinyl ether).

[0027] Polymers having a fluorine-containing alicyclic structure ontheir main chains are also obtained by copolymerization of a monomerhaving a fluorine-containing alicyclic structure such asperfluoro(2,2-dimethyl-1,3-dioxole) with a fluorine-containing monomerhaving at least two polymerizable double bonds such as perfluoro(allylvinyl ether) and perfluoro(butenyl vinyl ether).

[0028] As examples of the above-mentioned polymer having afluorine-containing alicyclic structure, those having a repeating unitselected from the following formulae (I) to (IV) are mentioned. Part ofthe fluorine atoms in such polymers having fluorine-containing alicyclicstructure may be substituted with chlorine atoms for the purpose ofincrease in refractive index.

[0029] [in the above formulae (I) to (IV), l is from 0 to 5, m is from 0to 4, n is from 0 to 1, l+m+n is from 1 to 6, each of o, p and q is from0 to 5, o+p+q is from 1 to 6, R is F or CF₃, R₁ is F or CF₃, R₂ is F orCF₃, X₁ is F or Cl, and X₂ is F or Cl].

[0030] As the polymer having a fluorine-containing alicyclic structure,polymers having a cyclic structure on their main chains are preferred.Those containing a polymeric unit having a cyclic structure in an amountof at least 20 mol %, preferably at least 40 mol % are preferred in viewof transparency and mechanical properties.

[0031] The material (b) is at least one material which differs from thefluoropolymer (a) in refractive index by at least 0.001. It may have ahigher refractive index or a lower refractive index than thefluoropolymer (a). In optical fibers, it is usual to use a materialhaving a higher refractive index than the fluoropolymer (a).

[0032] As the material (b), low-molecular weight compounds, oligomersand polymers containing an aromatic ring such as a benzene ring, ahalogen atom such as chlorine, bromine or iodine, or a bonding groupsuch as an ether bond are preferred. Further, the material (b) is amaterial which substantially has no C—H bond for the same reason as thefluoropolymer (a). The difference in refractive index between thefluoropolymer (a) and the material (b) is preferably at least 0.005.

[0033] The oligomeric or polymeric material (b) may be an oligomer or apolymer of such a monomer constituting the fluoropolymer (a) asdescribed above, which differs from the fluoropolymer (a) in refractiveindex by at least 0.001. Such a monomer is selected from those whichform a polymer which differs from the fluoropolymer (a) in refractiveindex by at least 0.001. For example, it is possible to use two kinds offluoropolymers (a) having different refractive indices and distributeone polymer (a) in the other polymer (a), as the material (b).

[0034] The material (b) preferably has a solubility parameter within7(cal/cm³)^(1/2) of that of the matrix. A solubility parameter is aproperty value which is a measure of the miscibility between materials.The solubility parameter δ is represented by the formula δ=(E/V)^(1/2),wherein E is the cohesive energy of a molecule of material, and V is themolar volume.

[0035] As a low-molecular weight compound, halogenated aromatichydrocarbons having no hydrogen atom bonded to a carbon atom may bementioned. Halogenated aromatic hydrocarbons containing only fluorineatoms as halogen atoms, and halogenated aromatic hydrocarbons containinga fluorine atom and another halogen atom are preferred in view of themiscibility with the fluoropolymer (a). Among such halogenated aromatichydrocarbons, those having no functional group such as a carbonyl groupor a cyano group are more preferred.

[0036] As such a halogenated aromatic hydrocarbon, a compoundrepresented by the formula Φ_(r)-Z_(b) [wherein Φ_(r) is a b valentfluorinated aromatic ring residue having fluorine atoms substituted forall the hydrogen atoms, and Z is a halogen atom other than fluorine,—Rf, —CO—Rf, —O—Rf or —CN, wherein Rf is a perfluoroalkyl group, apolyfluoroperhaloalkyl group or a monovalent Φ_(r), and b is 0 or aninteger of at least 1] may, for example, be mentioned. As the aromaticring, a benzene ring or a naphthalene ring may be mentioned. The carbonnumber of a perfluoroalkyl group or a polyfluoroperhaloalkyl group as Rfis preferably at most 5. As a halogen atom other than fluorine, achlorine atom and a bromine atom are preferred.

[0037] As specific compounds, for example,1,3-dibromotetrafluorobenzene, 1,4-dibromotetrafluorobenzene,2-bromotetrafluorobenzotrifluoride, chloropentafluorobenzene,bromopentafluorobenzene, iodopentafluorobenzene, decafluorobenzophenone,perfluoroacetophenone, perfluorobiphenyl, chloroheptafluoronaphthaleneand bromoheptafluoronaphthalene may be mentioned.

[0038] As the polymeric or oligomeric material (b) among those havingthe above-mentioned repeating units (I) to (IV), fluoropolymers having adifferent refractive index from the fluoropolymer (a) to be used incombination (for example, a combination of a fluoropolymer containingfluorine atoms only as halogen atoms with a fluoropolymer containingfluorine atoms and chlorine atoms, and a combination of two kinds offluoropolymers obtained by polymerizing at least two monomers ofdifferent kinds in different proportions) are preferred.

[0039] Further, in addition to the above-mentioned fluoropolymers havinga cyclic structure on their main chains, oligomers of monomerscontaining no hydrogen atom such as tetrafluoroethylene,chlorotrifluoroethylene, dichlorodifluoroethylene, hexafluoropropyleneand perfluoroalkyl vinyl ether, and co-oligomers of at least two ofthese monomers may be used as the material (b). Further,perfluoropolyethers having a structural unit —CF₂CF(CF₃)O— or—(CF₂)_(n)O— (wherein n is an integer of from 1 to 3) may be used. Themolecular weights of the oligomers are selected within such a range ofmolecular weight that they are amorphous, and are preferably from 300 to10,000 in terms of number-average molecular weight. In view of easydiffusion, it is more preferred that the number-average molecularweights are from 300 to 5000.

[0040] The particularly preferable material (b) is achlorotrifluoroethylene oligomer since it is excellently compatible withthe fluoropolymer (a), particularly with a fluoropolymer having a cyclicstructure on its main chain. By virtue of its good compatibility, it ispossible to easily mix the fluoropolymer (a), particularly thefluoropolymer having a cyclic structure on its main chain with achlorotrifluoroethylene oligomer by hot-melting them at 200 to 300° C.It is also possible to homogeneously mix them by dissolving them in afluorine-containing solvent and then removing the solvent. The preferredmolecular weight of a chlorotrifluoroethylene oligomer is from 500 to1500 in terms of the number-average molecular weight.

[0041] The optical plastic material of the present invention is mostpreferably a graded-refractive-index optical fiber. In the opticalfiber, the material (b) is so distributed in the fluoropolymer (a) as tohave a concentration gradient in the direction of from the center to theperiphery. Preferably, it is an optical fiber wherein the material (b)is a material having a higher refractive index than the fluoropolymer(a), and the material (b) is so distributed as to have such aconcentration gradient that the concentration of the material (b)decreases in the direction of from the center of the optical fiber tothe periphery. In some cases, an optical fiber wherein the material (b)is a material having a lower refractive index than the fluoropolymer(a), and the material (b) is so distributed as to have a concentrationgradient that the concentration of the material (b) decreases in thedirection of from the periphery of the optical fibers the center, isalso useful. A light transmission medium such as the former opticalfiber is usually produced by arranging the material (b) at the centerand diffusing the material (b) toward the periphery. A lighttransmission medium such as the latter optical fiber is produced bydiffusing the material (b) from the periphery toward the center.

[0042] A light transmission medium which is the optical plastic materialof the present invention has a transmission loss per 100 m of at most1000 dB at wavelengths of from 700 to 1,600 nm. Particularly, when afluoropolymer having an alicyclic structure on its main chain is used,it has a transmission loss per 100 m of at most 50 dB. It is quiteadvantageous that the transmission loss is at such a low level atrelatively long wavelengths of from 700 to 1,600 nm. Namely, it hasadvantages that since it is available to the same wavelength as vitreoussilica optical fiber, it can be connected to a vitreous silica opticalfiber without any difficulties, and that a cheaper light source can beused as compared with the case of conventional plastic optical fiberswhich are available only to light at wavelengths shorter than from 700to 1,600 nm.

[0043] In production of the optical plastic material of the presentinvention, the molding of the resins and the formation of the gradedrefractive index may be carried out simultaneously or separately. Forexample, the optical plastic material of the present invention may be soproduced by spinning or extrusion molding that a graded refractive indexis formed at the same time as formation of a graded refractive index. Itis also possible to form a graded refractive index after molding theresins by spinning or extrusion molding. Further, it is possible toproduce a preform (body material) having a graded refractive index andthen form (for example spin) the preform into an optical plasticmaterial such as an optical fiber. As described above, the opticalplastic material of the present invention also means such a preformhaving a graded refractive index.

[0044] As a method of producing the optical plastic material of thepresent invention, for example, the following methods (1) to (7) may bementioned. However, the present invention is not limited to thesemethods. The method (1) is particularly preferred.

[0045] (1) A method which comprises melting the fluoropolymer (a),injecting the material (b) or a fluoropolymer (a) containing thematerial (b) at the center of the melt of the fluoropolymer (a), andthen molding the melt while or after diffusing the material (b).

[0046] In this case, the material (b) may be injected at the center notonly so as to form only one layer but also so as to form multiplelayers. The molding is carried out by melt-extrusion, which is suitablefor forming a rod-like body material such as a preform of an opticalfiber, or by melt-spinnig, which is suitable for forming an opticalfiber.

[0047] (2) A method which comprises dip-coating the material (b) or thefluoropolymer (a) containing the material (b) on a core formed from thefluoropolymer (a) by melt spinning or drawing.

[0048] (3) A method which comprises forming a hollow tube of thefluoropolymer (a) by using a rotating glass tube or the like, filling inthe polymer tube with a monomer phase which gives the material (b) orthe fluoropolymer (a) which contains the material (b), and thenpolymerizing the monomer phase while rotating the polymer tube at a lowspeed.

[0049] In the case of interfacial gel polymerization, at thepolymerization step, the tube of the fluoropolymer (a) swells up in themonomer phase and forms a gel phase, and the monomer molecules arepolymerized while preferentially diffusing in the gel phase.

[0050] (4) A method wherein two kinds of monomers with differentreactivities, one of which is a monomer which forms the fluoropolymer(a), and the other is a monomer which forms the material (b), are used,and the polymerization reaction is carried out so that the compositionalproportion of the resulting fluoropolymer (a) to the resulting material(b) varies continuously in the direction from the periphery to thecenter.

[0051] (5) A method which comprises hot-drawing or melt-extruding amixture of the fluoropolymer (a) and the material (b) obtained byhomogeneously mixing them or by homogeneously mixing them in a solventand then removing the solvent upon evaporation, into fibers, and then(or immediately after the formation of the fibers) bringing the fibersinto contact with an inert gas under heating to evaporate the material(b) from the surface and thereby forming a graded refractive index. Or,a method wherein after the formation of the fibers, the fibers areimmersed in a solvent which does not dissolve the fluoropolymer (a) butdissolves the material (b) so as to dissolve out the material (b) fromthe surface of the fibers so that a graded refractive index is formed.

[0052] (6) A method which comprises coating a rod or a fiber of thefluoropolymer (a) with only the material (b) which has a smallerrefractive index than the fluoropolymer (a) or with a mixture of thefluoropolymer (a) and the material (b), and then diffusing the material(b) by heating to form a graded refractive index.

[0053] (7) A method which comprises mixing a high-refractive-indexpolymer and a low-refractive-index polymer by hot-melting or in a stateof a solution containing a solvent, and diffusing them in each otherwhile (or after) multilayer-excluding in a state that each has adifferent mixing ratio, to eventually obtain a fiber having a gradedrefractive index. In this case, the high-refractive-index polymer may bethe fluoropolymer (a), and the low-refractive-index polymer may be thematerial (b). The high-refractive-index polymer is the material (b), andthe low-refractive-index polymer is the material (b).

[0054] In the present invention, by virtue of the application of anamorphous fluoro resin to various plastic light transmission medium suchas a graded-refractive-index optical fiber, a graded-refractive-indexoptical waveguide and a graded-refractive-index rod lens, it is possibleto transmit light ranging from ultraviolet light to near infrared lightwith a quite low loss.

[0055] A graded-refractive-index optical fiber is particularly suitablefor optical communication over short distances in spite of its largediameter since it is flexible and it is easy to form branches andjunctions or it. However, no practical optical fiber with a low loss hasbeen proposed so far. The present invention provides a practical opticalfiber with a low loss for optical communication over short distances.

[0056] The light transmission medium of the present invention provides aplastic light transmission medium having heat resistance, chemicalresistance, humidity resistance and nonflammability enough to withstandsevere use conditions in an engine room of an automobile, an OAequipment, a plant and an electrical appliance. Thegraded-refractive-index optical plastic material of the presentinvention can be used not only as an optical fiber but also as a flat orrod lens. In such a case, by increasing or decreasing the-refractiveindex from the center to the periphery, it can function as a convex lensor a concave lens.

BRIEF DESCRIPTION OF DRAWING

[0057]FIG. 1 is the transmittance of polymer A.

BEST MODE FOR CARRYING OUT THE INVENTION

[0058] The present invention will be described in detail with referenceto Examples. However, it should be understood that the present inventionis by no means restricted to such specific Examples.

SYNTHESIS EXAMPLE 1

[0059] 35 g of perfluoro(butenyl vinyl ether) [PBVE], 5 g of1,1,2-trichlorotrifluoroethane (R113), 150 g of deionized water and 90mg of ((CH₃)₂CHOCOO)₂ as a polymerization initiator, were put in apressure glass autoclave of an internal volume of 200 ml. The atmospherein the autoclave was replaced by nitrogen three times, and suspensionpolymerization was conducted at 40° C. for 22 hours, yielding 28 g of apolymer having a number-average molecular weight of about 1.5×10⁵(hereinafter referred to as polymer A).

[0060] The intrinsic viscosity [η] of polymer A, measured inperfluoro(2-butyltetrahydrofuran) [PBTHF] at 30° C., was 0.50. The glasstransition point of polymer A was 108° C., and it was a tough,transparent and glassy polymer. The 10% thermal decompositiontemperature was 465° C., the solubility parameter was 5.3 cal/cm³, andthe refractive index was 1.34. FIG. 1 illustrates the transmittance ofpolymer A.

SYNTHESIS EXAMPLE 2

[0061] Perfluoro(2,2-dimethyl-1,3-dioxole) [PDD] and tetrafluoroethylenein the weight ratio of 80:20 were radical polymerized, and thereby apolymer having a glass transition point of 160° C. and a number-averagemolecular weight of about 5×10⁵ (hereinafter referred to as polymer B)was obtained. Polymer B was colorless and transparent, and had arefractive index of 1.3 and a high transmittance.

[0062] PDD and chlorotrifluoroethylene (CTFE)in the weight ratio of75:25 were radical polymerized, and thereby a polymer having a glasstransition point of 150° C. and a number-average molecular weight ofabout 3×10⁵ (hereinafter referred to as polymer C) was obtained. PolymerC was colorless and transparent, and had a refractive index of 1.4 and ahigh transmittance.

SYNTHESIS EXAMPLE 3

[0063] 8 g of PBVE, 2 g of PDD, 10 g of PBTHF and 20 mg of((CH₃)₂CHOCOO)₂ as a polymerization initiator were put in a pressureglass ampoule of an internal volume of 50 ml. The atmosphere in theampoule was replaced by nitrogen three times, and polymerization wasconducted at 40° C. for 20 hours, yielding 6.7 g of a transparentpolymer having a number-average molecular weight of about 2×10⁵(hereinafter referred to as polymer D).

[0064] The glass transition point of polymer D was 157° C., therefractive index was 1.32, and the content of PDD polymeric unitdetermined by measuring the absorbance at 1930 cm⁻¹ on the IR spectrum,was 12 wt %.

[0065] 2 g of PBVE, 8 g of PDD, 10 g PBTHF and 20 mg of ((CH₃)₂CHOCOO)₂as a polymerization initiator, were put in a pressure glass ampoule ofan internal volume of 50 ml. The ampoule was freeze-degassed threetimes, and then polymerization was conducted at 30° C. for 20 hours.Thereby, 7 g of a transparent polymer having a number-average molecularweight of about 3×10⁵ (hereinafter referred to as polymer E) wasobtained.

[0066] The glass transition point of polymer E was 210° C., therefractive index was 1.29, the content of PDD polymeric unit determinedby measuring the absorbance at 1930 cm⁻¹ on the IR spectrum, was 82 wt%.

EXAMPLE 1

[0067] Polymer A obtained by the above Synthesis was dissolved in PBTHFsolvent, and then 12 wt % of 1,3-dibromotetrafluorobenzene (DBTFB),which had a refractive index of 1.52 and was different from polymer A insolubility parameter by 3.2 cal/cm³, was added to obtain a mixedsolution. From the solution, the solvent was removed, to obtain atransparent mixed polymer (hereinafter referred to as polymer F).

[0068] Polymer A was melted, and melt-spinning was conducted at 300° C.while the melt of polymer F was injected at the center of the melt ofpolymer A, thereby an optical fiber having a refractive index graduallydecreasing in the direction of from the center to the periphery, wasobtained.

[0069] The light transmission property of the optical fiber thusobtained was 300 dB/km at 780 nm, and 130 dB/km at 1550 nm. The opticalfiber was confirmed to be capable of transmit light ranging from visiblelight to near infrared light satisfactorily.

EXAMPLE 2

[0070] 40 g of PBVE and 500 ml of ((CH₃)₂CHOCOO)₂ as a polymerizationinitiator were introduced into glass tube. After the tube wasfreeze-degassed, polymerization was conducted while the glass tube wasrotated at a high speed. The hollow tube thus synthesized was removedfrom the glass tube to obtain a tube of a polymer having anumber-average molecular weight of about 1×10⁵. The tube was chargedwith 20 g of PBVE, 2 g of DBTFB as a high-refractive-index material and200 ml of ((CH₃)₂CHOCOO)₂ as a polymerization initiator, and thensealed, and polymerization was conducted while the tube was rotated at alow speed.

[0071] At the polymerization step, the polymer of the tube swells up inthe monomer phase, forming a gel phase. The polymerization in the gelphase is promoted by the gel effect, and the polymer phase is formedfrom the periphery. At this time, the monomer molecules preferentiallydiffuse in the gel phase since monomer molecules are smaller in sizethan molecules of the high-refractive-index material, and thepolymerization proceeds with the high refractive index materialconcentrated at the center, to form such a graded refractive index thatthe refractive index gradually decreases from the center to theperiphery. The preform thus obtained as hot-drawn to obtain agraded-refractive-index optical fiber.

[0072] The light transmission property of the optical fiber thusobtained was 500 dB/km at 650 nm, and 150 dB/km at 1550 nm. The opticalfiber was confirmed to be capable of transmitting light ranging fromvisible light to near infrared light satisfactorily.

EXAMPLE 3

[0073] A core of 30 μ was prepared from polymer D obtained in the aboveSynthesis. On the other hand, a solution containing polymer D at aconcentration of 1 wt % in PBTHF solvent (hereinafter referred to assolution D) was prepared. Similarly, a solution containing 1 wt % ofpolymer E in PBTHF solvent (hereinafter referred to as solution E) wasprepared. Solution D was dip-coated on the core of polymer D at adrawing rate of 6 cm, and then dried at 180° C. It was found that thediameter of the core of polymer D increased by 100 nm.

[0074] Dip coating and drying were repeated 500 times while adding a{fraction (1/250)} portion by weight of solution E to the solution Deach time. Finally, solution E at a concentration of 10 wt % wasdip-coated and dried repeatedly five times, and it was dried at 180° C.for 2 hours. Thereby, an optical fiber having a diameter of about 600 μwherein the refractive index gradually decreased in a direction of fromthe core to the periphery, was obtained.

[0075] The light transmission property of the optical fiber thusobtained was 1050 dB/km at 650 nm, 460 dB/km at 950 nm, and 130 dB/km at1300 =m. The optical fiber was confirmed to be capable of transmittinglight ranging from visible light to near infrared light satisfactorily.

EXAMPLE 4

[0076] Equal weights of polymer B and polymer C synthesized above weredissolved in PBTHF solvent and mixed. From the resulting solution, thesolvent was removed to obtain a transparent polymer mixture (B+C). Amelt of polymer mixture (B+C) was poured inside a melt of polymer B, andfurther a melt of polymer C was injected at the center, while being meltspun, to obtain an optical fiber which has a refractive index graduallydecreasing from the center to the periphery.

[0077] The light transmission property of the optical fiber thusobtained was 550 dB/km at 650 nm, and 130 dB/km at 1550 nm. The opticalfiber was confirmed to be capable of transmitting light ranging fromvisible light to near infrared light satisfactorily.

EXAMPLE 5

[0078] An optical fiber was obtained in the same manner as in Example 1except that 30 wt % of a CTFE oligomer having a number-average molecularweight of 800 was used instead of 12 wt % of DBTFB. The refractive indexof the oligomer was 1.41, and the difference in solubility parameterbetween the oligomer and the polymer A was 1.4 cal/cm³. The opticalfiber thus obtained had such a refractive index as decreased graduallyfrom the center to the periphery.

[0079] The light transmission property of the optical fiber was 280dB/km at 780 nm, and 120 dB/km at 1550 nm. The optical fiber wasconfirmed to be capable of transmitting light ranging from visible lightto near infrared light satisfactorily.

EXAMPLE 6

[0080] 50 Parts of PDD having a reactivity ratio r¹ (the ratio of therate of production of PDD homopolymer to the ratio of production ofPDD/PBVE copolymer) of 1.9, 50 parts of PBVE having a reactivity ratior² (the ratio of the rate of production of PBVE homopolymer to the ratioof production of PDD/PBVE copolymer) of 0.19, and 1 part ofdialkoxyacetophenone as a photoinitiator, dissolved in 5 parts ofHCFC25, were introduced in a glass ampoule. After the ampoule wasfreeze-degassed three times, photopolymerization was conducted by usinga low-pressure mercury lamp. Thereby, a preform having a gradedrefractive index with a refractive index at the periphery of 1.31 and arefractive index at the center of 1.33, was obtained. It was hot-drawnto obtain a graded-refractive-index optical fiber.

[0081] The light transmission property of the optical fiber was 320dB/Km at 650 nm, and 250 dB/Km at 1550 nm. The optical fiber wasconfirmed to be capable of transmitting light ranging from visible lightto ultraviolet light satisfactorily.

EXAMPLE 7

[0082] 85 Parts of polymer A and 15 parts of DBTFB were melt-mixed andformed into a rod. The rod was hot-drawn at 200° C. to obtain a fiber.At this time, after emerged from the hot drawing region, the fiber waspassed through an electric oven of 1 m long at 120° C. In the electricoven, a flow of dry air preliminary heated to 120° C. was made toevaporate DBTFB from the surface of the fiber and thereby obtain anoptical fiber having a graded refractive index.

[0083] The light transmission property of the optical fiber was 420dB/km at 650 nm, 250 dB/km at 780 nm, and 110 dB/km at 1300 nm. Theoptical fiber was confirmed to be capable of transmitting light rangingfrom visible light to near infrared light satisfactorily.

EXAMPLE 8

[0084] 90 Parts of PBVE and 10 parts of CTFE were polymerized to obtaina polymer having a number-average molecular weight of about 2×10⁵(hereinafter referred to as polymer F). Polymer F was melt-mixed with aCTFE oligomer having a number-average molecular weight of 800homogeneously, and the melt mixture was formed into a rod containing theoligomer in an amount of 20 wt %.

[0085] The rod was hot-drawn into a fiber of 500 μ diameter. The fiberwas passed through ethanol to dissolve out the CTFE oligomer, and thenpassed through a cylindrical hot oven at 20° C. with a residence time ofabout 10 seconds for drying. Thereby, a graded-refractive-index opticalfiber with a refractive index at the periphery of 1.36 and a refractiveindex at the center of 1.38, was obtained.

[0086] The light transmission property of the optical fiber thusobtained was 250 dB/Km at 650 nm, and 150 dB/Km at 1550 nm. The opticalfiber was confirmed to be capable of transmitting light ranging fromvisible light to ultraviolet light satisfactorily.

EXAMPLE 9

[0087] Polymer C was spun at 270° C. by a extrusion method, and theresulting fiber was immediately passed through a hexafluoropropyleneoxide (HFPO) oligomer (number-average molecular weight 2100) heated to220° C. so that the residence time would be 3 minutes. As a result, theHFPO oligomer diffused and penetrated into the fiber, thereby, anoptical fiber having an outer diameter of 600 μ and a refractive indexcontinuously varying in the direction of from the periphery to thecenter, was obtained. The refractive index at the periphery was 1.34,and the refractive index at the center was 1.35.

[0088] The light transmission property of the optical fiber thusobtained was 300 dB/Km at 650 nm, and 130 dB/Km at 1550 nm. The opticalfiber was confirmed to be capable of transmitting light ranging fromvisible light to ultraviolet light satisfactorily.

EXAMPLE 10

[0089] A polymer having a PDD content of 20 wt % and a number-averagemolecular weight of about 1×10⁵ (hereinafter referred to as polymer G)and a polymer having a PDD content of 60 wt % and a number-averagemolecular weight of about 5×10⁵ (hereinafter referred to as polymer H)were synthesized by polymerization of PDD and PBVE. The refractiveindices were 1.33 for polymer G and 1.31 for polymer H, respectively.

[0090] Polymers G and H were dissolved inperfluorotributylamine/perfluorooctane=20/80 (weight ratio),respectively, so that the polymer concentrations would be 20 wt %. Then,the resulting solutions were mixed in the ratios shown in Table 1 toprepare 11 kinds of solutions, and they were heated to evaporate part ofthe solvent. Thereby gel solutions of about 3000 cP were obtained. The11 kinds of gels having different mixing ratios were extrudedconcentrically by using a multilayer nozzle into a multilayer fiber,while being heated at 80° C. The fiber was passed through a hot oven(about 150 to 200° C.) in which air flow was made to remove the residualsolvent. Thereby, a graded-refractive-index fiber was obtained.

[0091] The light transmission property of the optical fiber thusobtained was 350 dB/km at 650 nm, 150 dB/km at 950 nm, and 120 dB/km at1300 nm. The optical fiber was confirmed to be capable of transmittinglight ranging from visible light to near infrared light satisfactorily.

COMPARATIVE EXAMPLE

[0092] As for a graded-refractive-index plastic optical fiber, the lighttransmission loss of PMMA was about 400 dB/km at 650 nm, and thetransmission losses at wavelengths of 780 nm, 1300 nm and 1550 nm wereso large that it was impractical as a light transmission medium.

[0093] A stepped-refractive-index plastic optical fiber having a coreand a cladding made of fluoro resins is reported to be capable oftransmitting light ranging from visible light to near infrared light buthas a light transmission loss of about 300 dB/km.

[0094] By contrast, the graded-refractive-index transparent fluoro resinoptical fiber of the present invention is capable of transmit lightranging from visible light to near infrared light with extremely lowlosses. TABLE 1 Polymer G Polymer H 100 parts 0 part 81 19 64 36 49 5136 64 25 75 16 84 9 91 4 96 1 99 0 100

claims:
 1. A graded-refractive-index optical plastic material composedof an amorphous fluoropolymer (a) which substantially has no C—H bondand at least one material (b) which differs from the fluoropolymer (a)in refractive index by at least 0.001, wherein the material (b) isdistributed in the fluoropolymer (a) so as to have a concentrationgradient in a specific direction.
 2. The optical plastic materialaccording to claim 1, wherein the fluoropolymer (a) is a fluoropolymerhaving a cyclic structure on its main chain.
 3. The optical plasticmaterial according to claim 2, wherein the fluoropolymer having a cyclicstructure on its main chain is a fluoropolymer having afluorine-containing alicyclic structure on its main chain.
 4. Theoptical plastic material according to claim 3, wherein the fluoropolymerhaving a fluorine-containing alicyclic structure on its main chain has arepeating unit selected from the following formulae (I) to (IV):

[in the above formulae (I) to (IV), l is from 0 to 5, m is from 0 to 4,n is from 0 to 1, l+m+n is from 1 to 6, each of o, p and q is from 0 to5, o+p+q is from 1 to 6, R is F or CF₃, R₁ is F or CF₃, R₂ is F or CF₃,X₁ is F or Cl, and X₂ is F or Cl].
 5. The optical plastic materialaccording to claim 1, wherein the fluoropolymer (a) has a melt viscosityof from 103 to 105 poise at a melt temperature of from 200 to 300° C. 6.The optical plastic material according to claim 1, wherein the material(b) substantially has no C—H bond.
 7. The optical plastic materialaccording to claim 1, wherein the material (b) differs from thefluoropolymer (a) in solubility parameter by at most 7(cal/cm³)^(1/2).8. The optical plastic material according to claim 1, wherein thematerial (b) has a molecular weight of from 300 to
 5000. 9. The opticalplastic material according to claim 1, wherein the material (b) is achlorotrifluoroethylene oligomer.
 10. The optical plastic materialaccording to claim 1, which is a graded-refractive-index optical fiber.11. A graded-refractive-index optical fiber which is the optical plasticmaterial according to claim 1, wherein the material (b) is sodistributed as to have a concentration gradient in a direction of fromthe center to the periphery.
 12. The graded-refractive-index opticalfiber according to claim 11, wherein the material (b) has a higherrefractive index than the fluoropolymer (a), and is so distributed as tohave such a concentration gradient that the concentration decreases in adirection of from the center to the periphery.
 13. A method forproducing a graded-refractive-index optical plastic material, whichcomprises melting an amorphous fluoropolymer (a) which substantially hasno C—H bond, injecting at least one material (b) which differs inrefractive index by at least 0.001 from the fluoropolymer (a) or thefluoropolymer (a) containing the material (b) at the center of the meltof the fluoropolymer (a), and molding the melt while or after diffusingthe material (b) to form a region wherein the refractive index variescontinuously.
 14. The method according to claim 13, wherein thefluoropolymer (a) is a fluoropolymer having a cyclic structure on itsmain chain.
 15. The method according to claim 13, wherein the material(b) substantially has no C—H bond.
 16. The method according to claim 13,wherein the molding is conducted by melt extrusion molding or by meltspinning molding.